Pearlitic cast iron and method of producing the same



y 1933- H. A}. MITCHELL EH Ag. 199M334 PEARLITIC CAST IRON AND METHOD OF PRODUCING THE SAME Filed Jan. 8, 1930 gwucmtom 'WAMW -nification 100). Examination of Figure 1 Patented May 23, 1933 UNITED STATES PATENT OFFICE H. ALTON MITCHELL AND JAMES L. GAWTHON, JR, 0]? COLUMBUS, OHIO, ASSIGNORS TO THE BONNEY-FLOYD COMPANY, OF COLUMBUS, OHIO, A CORPORATION OF OHIO PEARLITIG CAST IRON AND METHOD OF PRODUCING THE SAME Application filed January 8, 1930. Serial No. 419,420.

This invention relates to the production of gray cast iron of essentially pearlitic structure and has'for its object the production of'castings of great strength, moderate Brinnell hardness, uniformity of structure, great resistance to frictional wear and abrasion and great stability of metallographic struc tural constituents at ordinary and elevated temperatures. a i

Normal good gray cast iron has the folimpurities. When an iron of such an average analyses 1s 1n the molten condition, the entire carbon content 1s m solution 1n the 1ron. When the molten iron is cast into a sand mould and begins to solidify, a portion of the carbon present is precipitated from combination with, or solution in, the iron into the free or graphitic condition, the balance remain ing in combination with the iron as carbide of iron.

If this molten iron be cast into asan'd mould and allowed to cool normally in the sand with no artificial acceleration or retardation of the rate of cooling, it would have a graphitic or free-carbon cont'entof'approximately 2.90 %-,-3.4O%.7

Microscoplc examination of the above men tioned lIOIl would reveal ametallographlcstructure, in a suitably-prepared and etched specimen, having esscntiallythc characteristics as shown in Figure 1 (approximate magshows the metal to have three structural constituents, the solid black, elongated, flakey areas, which are free or graphitic carbon, the hairlined or shaded, granular areas which are pcarlite and the solid white, granular areas, which are ferrite. Pear-lite is an intimate mechanical mixture offerrite, or pure iron and cementite, the carbide of iron. The shaded areas of pearliteand the white areas of ferrite together constitute a matrix, in

carbon are imbedded, in some cases crossing the grain boundaries of the pearlite and ferrite grains. From the above, it is at once apparent that such a gray cast iron as the above is, in reality, steel of 35% carbon content, with particles of graphite imbedded in it. v

Since graphite has practically no strengt the strength of a piece of cast iron must depend on the matrix, which is steel; and must have roperties commensurate with. the general c aracteristics of steel, one of which is itsdependence on the amount of carbon present for its strength. The combined carbon in the steel matrix of a piece of cast iron thus determines in a directly proportional manner the strength of that piece of cast iron. But since, as above noted, graphites strength is a negligible quantity, the graphite particles are detrimental to strength in direct which the particles or flakes of graphitic 1 proportion to their quantity, since they 00- cupy space which, but for their presence,

would be occupied by the stronger materials of the matrix; and they break into and. disrupt the metallic continuity of the matrix, The strength of pearlite is ordinarily 125,000 pounds per square inch andthe strength of ferrite is 55,000 pounds per square inch.

From the above well known facts, it is readily seen that for two pieces of iron of the same total carbon content, but with different combined carbon contents that iron with the larger combined carbon content will be decidedly the stronger. For the iron with the smaller combined carbon-content willhav the greater graphitic carbon content, an

will have gained a greater percentage of weaker structural constituents (gra hit-e and ferrite) by the'loss of a part 0 the stronger constituent (pearlite). 1

It is broadly and generally known that if the combined carbon content of a cast iron be fixed at approximately .85%, a metal will result possessingl very excellent oharacieristics of strengt silience and resistance to abrasive and frictional wear, but moderate Brinnell hardness.

Such an iron will have a metallographic at f i good machineability, r

structure as shown in Figure 2 (approximate magnification, 100). Referring to Figure 2, it will be seen that there are but two structural constituents, a matrix of pearlite with finer and more evenly distributed graphite particles. Such an iron' is called pearlitic and the pearlite matrix is called an eutectoid structure.

Pearlitic iron is generally considered ideal for castings which are applied to services in which they are subjected to heavy duty, im-

pact, frictional and abrasive wear and para lower point, even below the critical temperature, of the iron, for an extended period of time, the solid iron carbide will decompose, forcing graphite into the precipitated free state, leaving the iron with which it was combined as ferrite. This. increase in graphitic carbon content proceeds by increase in the size ofthe original graphite particles and by formation of new and additional particles; and it is attended by actual increase in cubical dimensions of the casting, which is due to a decrease in the density of carbon as it changes from a state of chemical combination with the iron to its free'condition. In growing, we have found that cast iron is definitely and excessively weakened. By testing the transverse strength of two pieces of cast iron, poured from the same ladle full of metal, one of which was broken as cast, the other being subjected to an annealing temperature of 1450 F. for one hour before breaking, we have found that it is an ordinary expectation for cast iron to suffer the loss of half itsstrength by such subjection to heat. This is due to lack of stability of the combined carbon (iron carbide) of irons not provided-with an effective stabilizing influence for itscarbides.

We have discovered that chromiumand molybdenum, due to their peculiar alloying effect, produce an iron having great strength, toughness, resilience and stability of structural constituents at elevated temperatures,

together with tremendousresistance to wear at ordinary temperatures and at elevated temperatures. In our experiments, we have found that. by the use of chromium, particulady in con] unction with molybdenum,which' decidedly accentuates the effect of-the chromium, we are able to produce a metal with great resistance to decomposition of its carbides. We have made transverse tests of pieces of our metal poured from the same heat with and without annealing by the same treatment as above described as applied to ordinary cast iron. These tests showed that our metal in no instance lost more than one tenth its strength, while as above described, ordinary iron with no agent present to stabilize the strengthening carbides, lost fully half its strength.

The fact that we utilize preferably an electric furnace for melting the material comprising the alloy permits the temperature and chemical analysis to be very accurately controlled, and also admits of the attain? ing of high melting and superheating temperatures, which are essential in producing a finely divided and nodular graphite in the finished product. The pouring temperatures may vary between 2500 F. and 3000 F., or these tempertures are reached and obtained sometime during the melting and refining process.

We have also found that there are certain advantages in making pearlitic iron in producing first a metal with a hypereutectoid structure, which has massive or free cementite as a structural constituent, which is decomposed and eliminated by thermal treatment of the casting after it is poured and allowed to cool to ordinary temperatures. This allows the use of wider ranges in the analyses of the metal, and eliminates the troublesome and costly methods of preheating moulds used by previous investigators. The thermal treatment referred to has another beneficial effect, which is the relief of the shrinkage strains in the casting and in consequence, warpage or distortion of the casting during and after machining.

Thus it is an object of this invention to produce pearlitic cast iron wherein the iron during its process of manufacture is reduced to a molten condition and refined and superheated by the use of an electric .furnace, or is reduced to a molten condition by the use of some other type of furnace such as a cupola, air furnace or open hearth furnace and subsequently superheated and refined by the employment of an electric furnace.

It is another object of this invention to produce a cast iron alloy containing chromium and molybdenum, and through the medium of high temperatures applied at some timeduring the period in which the alloy is in a molten condition, to produce in the subsequent cold iron an esscntially'pearlitic structure.

As an illustration of the new alloy composition, the following table setsforth the relative proportions and ingredients there- Per cent Chromium And iron approximately sufficient to make 100%, except for impurities such as phosphorous, sulphur and the like which are incidental to manufacture.

We have also secured satisfactory results from an alloy composed of the following ingredients in substantially the proportions given:

Per cent Chromium .10 to 2.00 Nickel .01 to 5.00 Molybdenum .01 to 2.00 Carbon 1.75 to 3.75 Silicon .50 to. 3.00 Manganese .25 to 2.00

The remainder being iron, except for impurities such as phosphorous, sulphur, etc., which are incidental to the manufacture.

portions may be used:

Also the followingingredients in the pro- Per cent Iron approximately suflicient to make 100%.

Another example consists in the use of the following:

Per cent Nickel .01 to 5.00 Molybdenum .01 to 2.00 Carbon; 1.75 to 2.75 Silicon .50 to 3.00 Manganese .25 to 2.00

Iron approximately suflicient to make 100% In the manufacture of this product, the iron may be melted in a fuel fired furnace or an electric furnace, and is subsequently alloyed, refined and superheated in an electric furnace to a temperature varying ordinarily between 2500 F. and 3000 F. This results in a metal whose final constituents are found to comprise a matrix of earlite interspersed with finely divided even y disseminated particles of graphite of nodularshape (see Figure 4) and smaller size than those of ordinary gray (see Figure 3-) iron or even' ordinary pearlitic iron. The molybdenum and chromium are provided -in order to increase the stability of carbides at elevated temperatures, to raise the critical temperature, at which carbide decomposition proceeds with rapidity thereby further increasing the stae bility of the carbides of the iron at any'given temperature below said critical temperature and to strengthen and harden the matrix and' increase the resistance of the metal to shock,

impact, frictional and abrasive wear, fatigue and growth. For castings for special purposes, We have found the use of nickel in the percentages above specified is very advantageous due to its ability to strengthen the matrix in its pearlitic ferrite, and to add to the physical properties above described.

We prefer to use the electric furnace in melting the'materials comprising this alloy for' the reason that the temperature and chemical analysis may be very accurately controlled; but a cupola or other fuel fired furnace may be employed for the meltin and an electric furnace used subsequently or al loying, refining and superheating to the elevated temperatures which are essential in producing a finely divided and nodular graphite in the finished iron. As stated, these temperatures may vary from substantially 2500 F. to 3000 F., and are obtained at some time during the melting and refining process. What is claimed is: 1

1. An alloy iron which has been superheated in the molten condition at a temperature between 2550 F. to 3000 F. and containing .10% to 2.00% chromium, .10% to 2.00% molybdenum, 1.75% to 3.75% carbon, 50% to'3.00% silicon, 25% to 2.00 manganese,.the remainder being iron except for impurities such as phosphorus, sulphur and the like which are incidental to the manufacture, and having as its final constituents a matrix of essentially all pearlite interspersed with relatively finely divided particles of graphite, the chromium and molybdenum contents being sufiicient in amount to increase the staprise a matrix of pearlite interspersed With finely divided evenly disseminated particles of graphite of nodular shape and smaller size than those of ordinary gray iron, the carbon content being from 1.75% to 3.75%, there being chromium from 10% to 2.00%, molybdenum from ..10%, to 2.00%, 1 the balance substantially cast iron, said alloy ing elements being sufficient in amount to increase the stabillty of the carbide present at elevated temperatures, and such article of manufacture having been superheated in the molten condition at a temperature of from 2550 to 3000 F.

3. The method of manufacturing an alloy iron which consists in melting in an'electric furnace a ferrous alloy consisting of .10% to 2.00% chronium, .10% to 2.00% molybdenum, 1.75% to 3.75% carbon, 1.50% to 3.00% silicon, 25% to 2.00% manganese, the remainder being iron, except for incidentalim purities, the melting of such alloy within said uct wherein the final constituents thereof comprise essentially a matrix of pearlite interspersed with relatively finely divided particles of graphite.

5 4. The method of producing cast iron of pearlitic structure containing as essentials from .10% to 2.00% chromium and from .10% to 2.00% molybdenum, the balance substantially iron which comprises melting the same in a. cupola or other 'fuel fired furnace and alloying, refining and superheating in an electric furnace at a temperature between 2550 F. to 3000 F.

5. The method of producing alloy cast iron having final constituents which are essentially a matrix of pearlite'interspersed with fine- 1y divided particles of graphite, which consists in superheating at a temperature between 2550 F. and 3000 F. in an electric furnace a composition consisting of chromium .10% to 2.00%; molybdenum 10% to 2.00%, and the balance substantially iron.

In testimony whereof we afiix our signatures. H. ALTON MITCHELL.

JAMES L. CAWTHON, JR. 

